TGF-beta is a potent inhibitor of the proliferation of epithelial cells, and it has been widely assumed that this property is critical for the ability of TGF-beta to function as a tumor suppressor. In FY07, we have provided evidence that TGF-beta may suppress tumorigenesis through a novel mechanism that is independent of direct effects on cell proliferation. Using a breast cancer xenograft model, we have shown that endogenous TGF-betas can suppress tumorigenesis by regulating cancer stem cell dynamics. They do this by reducing the size of the putative cancer stem cell population and also by promoting differentiation of the highly proliferative cancer progenitor cells. Experimental blockade of TGF-beta response in this model system converted the gene expression profile and histology of the tumor from a differentiated luminal to a less differentiated basal state, which previous clinical breast cancer studies have shown to be associated with a poorer prognosis. We identified the transcriptional regulator Id1 as a critical downstream target of TGF-beta in regulating differentiation, and we propose that molecular changes that block differentiation can selectively block the tumor suppressor effects of TGF-beta, thereby allowing pro-progression activities to dominate, and thus contributing to the metastatic switch. Despite the dual role for TGF-beta as both tumor suppressor and tumor promoter in carcinogenesis, preclinical data from our lab and others has previously suggested that strategies to antagonize TGF-beta may selectively reduce the undesirable tumor promoting effects of this growth factor, while sparing the desirable effects on tumor suppression and normal homeostasis. Based on these promising preclinical results, a number TGF-beta antagonists are in early phase clinical trials for the treatment of advanced cancer. However, relatively little is known about how these agents work. In FY07, we have made substantial progress in understanding the mechanism of action of anti-TGF-beta antibodies in suppressing tumor progression and metastasis. To do this, we have used the 4T1 syngeneic mouse transplantation model of metastatic breast cancer, in combination with global and candidate gene expression analysis, molecular histology, immunophenotyping and immunodepletion approaches. We have found that a monoclonal anti-TGF-beta antibody (1D11 from Genzyme Corporation) suppresses tumorigenesis and metastasis through a combination of many small effects on multiple cellular compartments. Target cells include the tumor cell itself, components of the immune surveillance system, and the microvasculature. We propose that this distributed mechanism of action may be critical for the unexpectedly low toxicity of the agent. These data have important implications for clinical biomarker development and for understanding the biological basis of the selectivity of this class of TGF-beta antagonist in affecting the tumor and not the normal tissues.